Electrical stimulator devices are used to stimulate various types of organic tissue. For example, electronics may be interfaced with the nervous system of a human body through use of neurostimulators. A neurostimulator is a device that may be implanted into human tissue, and provides stimulation to neurons through electrical pulses. For such reasons, neurostimulators may be referred to as implanted pulse generators (IPGs).
A typical neurostimulator includes one or more electrodes. Some neurostimulators include arrays of electrodes configured in implantable devices. For example, FIG. 1 illustrates a typical system 100 for neurostimulation. The system 100 includes an implantable electrode array 102 which may be implanted in organic tissue 106. For example, the implantable electrode array 102 may be implanted in a human brain or near optical or auditory nerves. The system 100 also includes a controller 104 coupled to the implantable electrode array 102. The electrode controller 104 is often located outside of the body. In certain systems, however, the electrode controller 104 may also be implantable.
FIG. 2 illustrates one configuration of a electrode array 102. As illustrated, the electrode array 102 includes a plurality of electrodes 202a-d, and a current return 204. A dielectric backing 107 mechanically supports the array and helps directing the charge injection into the tissue. The electrode array 102 may be implanted so the electrodes are in contact with organic tissue 106. In most prior systems, the current return 204 is placed in a region relatively distal from the electrode array 102. Each of the electrodes is coupled to a power source which injects charge on the electrodes 202a-d. The charge is injected through the organic tissue 106 to the current return 204, thus stimulating the organic tissue 106 between the electrodes 202a-d and the current return 204. Typically, only the portions of organic tissue 106 directly proximate each of the electrodes 202a-d are stimulated.
In order to increase the stimulation resolution of an electrode array 102, additional electrodes 202 may be included in the electrode array 102. For example, in the case of a retinal prosthesis, the electrode array 102 may originally have included nine electrodes 202. In an effort to increase resolution, the electrode array 102 could eventually include sixty-four electrodes 202 and then two hundred electrodes 202 or more in later versions.
In prior stimulator systems, as the number of electrodes increases, for the same size device, each electrode has to be smaller. This causes the current density at the vicinity of the stimulating electrodes to grow accordingly, to a point in which further miniaturization may lead to current density magnitudes that can damage the tissue. Additionally an additional wire and an additional driver circuit are typically required for each electrode, and the wire needs to have a large enough section to allow the appropriate amount of current to flow. These wires make the implant bulkier, mechanically stiffer, and in general harder to conform to delicate anatomical features.